Method for the manufacture of agglomerable iron powder



Patented Apr. 8, 1952 METHOD FOR THE MANUFACTURE OF AGGLOMERABLE IRON POWDER Louis Neel, Grenoble, France, assignor to Societe dElectro-Chimie, dElectro-Metallurgie et des Acieries Electriques dUgine, Paris, France, a corporation of France No Drawing. Application March 28, 19.45,-'Serial No. 585,400. In France March 1, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires March 1, 1963 1 Claim. 1

for instance, advisable that the said powders should lend themselves easily to the agglomeration in the cold under pressures which are not too high and give rise, in this state, to the production of objects showing a sufiicient density. At the same time, the grains should have a contexture suitable for insuring to the object a good cohesion and the absence of friability.

Powders usually manufactured in the art through mechanical means, through electrolysis, through reduction of oxidized iron compounds meet these conditions only imperfectly. Thus, the compression of the said powders through application of loads of 5 to tons per square centimeter does not make it possible to obtain compressed goods having a density higher than 5.5 g. per cc. which corresponds to a proportion of voids reaching up to 28% of the total volume of the object, i. e. to a comparatively low filling 'coeflicient. Furthermore, such compressed goods are generally friable; when tested in the Brinell machine impressions of a large diameter are obtained which are due to the sliding of the grains around the ball or the diamond, which corresponds to a deficiency of cohesion in the material. The iron powders which are found in laboratories and which are usually prepared through reduction of iron oxalate yield results which are slightly better than those of the commercial powders, but the cost of powders prepared from iron oxalate is high for, on one hand, the starting product is expensive and, on the other hand, the reduction is eiiected at comparatively high temperatures requiring the use of special implements.

The applicant has already suggested to make iron powders through decomposition of iron formate followed by a reduction at comparatively objects having particularly high mechanical properties. Experience shows that both qualities: a high coercive force, on one hand, and a great facility of agglomeration, on the other hand, cannot usually be obtained simultaneously.

The present invention relates to a method for making an iron powder for which high values of the coercive force are not especially aimed at, this being the case for permanent magnets, but which lends itself particularly well to the manufacture, through agglomeration, of objects of any kind which must show high mechanical characteristic properties (strength, density, compactness and the like). The said powder yields through a simple compression compressed goods having a high density, of the order of '7 g. per 00., the proportion of voids being lower than about 10%, for pressures of 5 to 10 tons per square centimeter, without any friability and showing a high mechanical strength.

The method according to the invention essentially consists in starting from iron formate, in effecting a decomposition of the said body through heat in a neutral or reducing atmosphere and in proceeding simultaneously with or subsequently to the said decomposition to a very far-reaching reduction of the products of the said decomposition by means of a reducing gas at a temperature between 500 and 750 C.

Iron formate is used as a starting material for the production of the powder. As a matter of 'fact the applicant has revealed the surprlzing fact that the said compound makes it possible to obtain powders which are much better suitable for agglomeration than laboratory powders made through decomposition and reduction of an organic compound which, however, is very closely related, i. e. the oxalate. This result is obtained, moreover, by effecting the reduction at not very high temperatures, which is an interesting fact from the point of view of the cost. Furthermore, the powder prepared when starting from formate is, all other things being the same, decidedly purer than the powder prepared when starting from oxalate. The following table distinctly shows the differences of the results .as to the agglomeration between the oxalate method and the formate method according to the invention. The figures indicated therein relate to compressed goods obtained from two powders through a simple compression in the cold under a .pressure of 10 tons per square centimeter and g ve n bot cases und r consi eration therm- 3 portion of voids with respect to the total volume of the compressed piece.

The iron formate used as a starting material is prepared by any konwn means. The said formate is decomposed by heat and, simultaneously or subsequently, one proceeds to the reduction of the product of the said decomposition by means of a reducing gas, but while for obtaining powders with a high coercive force a reduction is effected which can be other than total at temperatures of the order of 300 0., according to the invention one proceeds to a very far-reaching reduction at a higher temperature between 500 and 750 C.

The examples summed up in the following table and relating to powders reduced when starting from iron formate at different temperatures reveal the variations of the results obtained in function of the reduction temperature and of the duration of said reduction. In the two latter columns one has indicated, on one hand, and values of the coercive force obtained in the different cases and, on the other hand, the proportions of voids in percent in compressed objects obtained through a simple compression in the cold under pressures of 10 tons per square centiject compresshours no use per cent The reduction grade to be obtained must be as high as possible. Practically, products contain- .ing no more than 1% of oxygen meet most of the usual requirements but the results become better and better as the said content is lowered. The desirable far-reaching reduction will be obtained by varying the nature of the reducin gas, its delivered quantity for its pressure. The reduction will .be the better the purer the gas which is used, the more rapid its flow or the higher its pressure, all other data being, the same. If necessary preliminary tests will make it possible to ascertain for each plant the best conditions which are to be adopted for a given result.

The reduction temperature should not exceed 750 C. Indeed, when the temperature is too high more complicated implements and more expensive fire-proof materials are to be used which increase the cost of the operation and of the obtained powder. Furthermore, when the temperature of 750 C. which is indicated as a maximum is nearly reached there is a probability of a commencement of sintering of the powder which complicated the manufacture and immediately diminishes the characteristic qualities 4 of the obtained product. On the contrary, it one works at temperatures lower than 500 C, the reduction will take place more slowly and, accordingly, be less complete for the same working time; thus powders will be obtained which possess a considerable coercive force but which will have a less high density when agglomerated in the cold. Finally, when working at a very low temperature as, for instance, at 300 C., it is necessary considerably to prolong the reduction time, which is not consistent with a low cost. In short, the reduction temperature will be chosen while taking into account the preceding considerations and according to the density or the filling coeificient which is to be obtained in the compressed product which it is intended to make.

A few non-limitative examples of a method for carrying out the invention are given hereunder:

Example I .-Iron formate has been decomposed and reduced at 550 C. in a current of dry and pure hydrogen flowing at the rate of 30 liters per gram of iron per hour under the ordinary pressure; after a reduction time of one hour an iron powder has been obtained which after having been compressed in the cold under a pressure of 10 tons per square centimeter has given a compressed product having a density of 7.1 g. per cc. Tested in the Brinell machine this compressed product has given an impression corresponding to Brinell units which shows that the grains possess a great cohesion.

Example II.-A similar operation has been eifected with the same flow'of hydrogen and under the same pressure but while proceeding to the reduction at a temperature of 650 C. Here also the compression of the powder in the cold under a pressure of 10 tons per square centimeter have produced a compressed product having a density of '7 g. per cc. and also showing a good cohesion.

Example III.Another operation has been effected with, this time, a reduction temperature of 550 0. as in Example I but a four times weaker flow of hydrogen. The density of the object compressed in the cold under a pressure of 10 tons per square centimeter is then only 5.5 instead of 7 g. per cc.

The powder made by the method according to the invention is particularly suitable for making any compressed objects agglomerated with or without a binding agent and which must show a good mechanical strength with an absence of fragility.

It is also suitable for making cores of selfinduction coils.

In a general manner the powder made according to such a method will require a pressure which is less high than the pressure required by ordinary powders for the obtention of the same density or for making it possible to obtain distinctly higher densities while using usual pressures.

The powder made according to the said method will also be used with advantage for making any sintered objects because, on one hand, the increase of density obtained on the objectwhich has been simply compressed in the cold results in a diminution of the shrinkage at the moment of the sintering; on the other hand, the fineness and the contexture of the grain and the possibility of obtaining high densities on the object compressed in the cold lead to an amelioration of the contacts between the grains which makes it possible to lower the sintering temperature.

be used alone or mixed with other metallic powders with or without a binding and insulating agent and according to the nature of the objects to be made.

I claim:

A method of making an exceedingly fine iron powder capable of forming by cold compression compressed products having a density of the order of at least 7 grams per 00., having a, proportion of voids less than about 10% for agglomerating pressures of 5 to 10 tons per square centimeter, which are non-friable and have high mechanical strength, which method consists in decomposing iron formate by heat and reducing the decomposed product with reducing gas at a temperature between 500- C. and 750 C. and at a great flow rate until the iron powder contains not over 1% of oxgen.

LOUIS NEEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,335,161 Ellis Mar. 30, 1920 1,982,689 Polydorofi Dec. 4, 1934 1,986,197 Harshaw Jan. 1, 1935 2,096,009 Schmid Oct. 19, 1937 2,119,489 Beer May 31, 1938 2,183,145 Michael et a1. Dec. 12, 1939 2,220,261 Michael Nov. 5, 1940 FOREIGN PATENTS Number Country Date 419,953 Great Britain .....1 Nov. 22, 1934 OTHER REFERENCES Modern Inorganic Chemistry, by Mellor, published by Longmans, Green and Co., 1925, page 588.

Organic Chemistry by Karrer, published by "Elsevier Amsterdam, 1938, distributed by Nordeman Pub. Co., Inc., N. Y., page 176.

A Course in Powder Metallurgy, by Baeza, published by the Reinhold Publishing Corp. 1943, pages 37-43 and 196-201. 

